Define WIN32_LEAN_AND_MEAN when including windows.h

The hilariously-named WIN32_LEAN_AND_MEAN slims down the number of
header files included in the already-massive `windows.h`. I know people
say Moore's law and precompiled headers and all that (well, we don't use
precompiled headers), but they kinda forgot about virtual machines, and
there's no reason not to define this and slim down the number of headers
anyway.

desktop_version/src/FileSystemUtils.cpp

@@ -15,6 +15,7 @@
 
 /* These are needed for PLATFORM_* crap */
 #if defined(_WIN32)
+#define WIN32_LEAN_AND_MEAN
 #include <windows.h>
 #include <shlobj.h>
 static int mkdir(char* path, int mode)